Conventionally, in order to give a good appearance, a chain casing which covers the entire chain travelling path or a half chain casing which covers the chain travelling path at the driving or tension side only has been utilized. Recently, many bicycles using no chain casing have been put on the market due to weight reduction and low manufacturing cost of such bicycles.
Many bicycles of multistage speed change have been sold. For multistage rear gear assemblies, a rear derailleur having a tension spring for applying a chain tension to the driving chain is used.
When the chain tension is excessive, the speed change efficiency is problematical. In some cases, due to construction of the rear derailleur, the chain at the slack side, in other words, the chain transported from the front gear to the rear gear and subjected to no load (a driving force) is guided by a guide roller at a chain guide, at which time the guide roller interferes with a larger diameter rear gear to generate sounds. Hence, in order to solve the above problem, the tension of the tension spring is set low.
However, when the chain tension of the chain at the slack side is made smaller, the chain tension at the tight side or driving side also is reduced. Hence, when a cyclist rides the bicycle having a half chain casing or no chain casing on a downward slope by not-pedalling or by pedalling to apply a small load to the chain, the chain may vertically vibrate upon bouncing of the bicycle. In this case, when part of the chain at the driving side is caught by, for example, a tread of a rear wheel at the bicycle, there is a risk that the chain will be taken into the rotating direction of the rear wheel, that is, toward a chain stay.
Particularly, in a bicycle called the mountain bicycle, provided with multistage front gears and multistage rear gears and having wheels with relatively high treads, when the driving chain is stretched across a low speed front gear and a low speed rear gear and the cyclist rides the bicycle on a downward slope without pedalling, the aforesaid problem is quite remarkable.